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Turkey and the European Union are locked in an explosive crisis that threatens to jeopardise Ankara's bid to join the bloc, as tensions rise ahead of an April 16 referendum on expanding Erdogan's powers.

Germany angrily warned Turkey on Sunday that President Recep Tayyip Erdogan had "gone too far" after he accused Chancellor Angela Merkel of using "Nazi measures" in an escalating diplomatic feud. Turkey and the European Union are locked in an explosive crisis that threatens to jeopardise Ankara's bid to join the bloc, as tensions rise ahead of an April 16 referendum on expanding Erdogan's powers.

The row erupted after authorities in Germany and other EU states refused to allow some Turkish ministers to campaign for a 'yes' vote on their soil, provoking a volcanic response from the Turkish strongman who said the spirit of Nazi Germany was rampant in Europe.

"When we call them Nazis they (Europe) get uncomfortable. They rally together in solidarity. Especially Merkel," Erdogan said in a televised speech. "But you are right now employing Nazi measures," Erdogan said referring to Merkel, pointedly using the informal "you" in Turkish.

"Against who? My Turkish brother citizens in Germany and brother ministers" who planned to hold campaign rallies for a 'yes' vote in next month's referendum, he said.

Germany's foreign minister Sigmar Gabriel branded Erdogan's comments "shocking". "We are tolerant but we're not stupid," he told the Passauer Neue Presse newspaper. "That's why I have let my Turkish counterpart know very clearly that a boundary has been crossed here." Germany, home to 1.4 million Turkish voters, hosts by far the largest Turkish diaspora community in the world but the partnership between Ankara and Berlin has been ripped to shreds by the current crisis.

"Yesterday (Saturday), Germany put its name under another scandal," presidential spokesman Ibrahim Kalin told CNN-Turk. He said the German ambassador had been summoned although this was not confirmed by Berlin.

The Turkish foreign ministry accused the German authorities "of the worst example of double standards" for allowing the pro-Kurdish protest while preventing Turkish ministers from campaigning there for a 'yes' vote. Many protesters carried symbols of the Kurdistan Workers' Party (PKK), listed as a terror organisation not just by Turkey but also the EU and the United States.

Ankara also reacted with indignation after Germany's intelligence chief said he was unconvinced by Turkish assertions that US-based Islamic preacher Fethullah Gulen was behind the failed July coup aimed at overthrowing Erdogan. Kalin said Europe was seeking to "whitewash" Gulen's group, while defence minister Fikri Isik said the comments raised questions about whether Berlin itself was involved in the putsch.